Method for determining the position of the rotor of an electrical machine

ABSTRACT

A method for determining the position of the rotor of an electric machine having multiple phases in relation to the stator. The change Δi in the time derivative I of the current I flowing through at least one of the power inputs is determined. The change Δi is caused by a change ΔU in the potential U on at least one of the power inputs of the electric machine. A measurement signal that is representative of a position of the rotor is determined from multiple simultaneously or successively determined changes Δi.

The invention relates to a method for determining the position of therotor of an electrical machine having several phase strands, in relationto the stator, and an apparatus for carrying out the method.

EP 1 005 716 B1 discloses a method for determining the position of therotor of an electrical machine, wherein the changes of the potential atthe star point caused by changes of the potential of the rotor at thecurrent supply inputs are utilized for determining a signal which isrepresentative for the rotor position.

The invention is based on the object of creating a novel method fordetermining the position of the rotor of an electrical machine which canbe carried without picking up an electrical value within the electricalmachine.

The method according to the invention which meets this object ischaracterized in that, for at least one of the current supply inputs ofthe electrical machine, a change Δi of the time derivation I of thecurrent flowing through the respective current supply input isdetermined as a result of a change ΔU of the potential U on at least oneof the current supply inputs of the electrical machine, and a signalrepresentative for the position of the rotor is determined from severalchanges Δi occurring simultaneously or successively.

In accordance with the invention, the determination of the position ofthe rotor takes place exclusively with the aid of currents which can bemeasured outside of the electrical machine and with changeable voltages.

Preferably, inductivity of one or more phase strands or/and a quotientof these inductivities at a given time are determined as representativemeasurement signals.

The change ΔU of the potential U takes place within such a short periodof time that voltages induced in the phase strands in the meantime andcurrents flowing through the current supply inputs do not noticeablychange.

The phase strands can be switched in a star configuration or/and in atriangular configuration.

Preferably, the change ΔU of the potential U from a potential zero pointto a direct voltage takes place, particularly a battery voltage, or viceversa.

In accordance with a particularly preferred embodiment, the change ΔU ofthe potential U takes place within the scope of a current supply of theelectrical machine through pulse width modulation (PWM). Thedetermination of the position now does not require an intervention inthe electrical machine itself, or in the manner of operation thereof.

The change Δi can be determined, for example, by means of a measuringtransformer which, for example, produces a voltage signal which may haveto be reinforced. For example, in the case of a voltage switch occurringwithin the scope of the pulse width modulation, a voltage leap resultsat the secondary winding of the measuring transformer which correspondsto the voltage leap during switching.

Several changes Δi caused by several potential changes ΔU can bedetermined for only one of the current supply inputs. The expenses formeasurement circuits are correspondingly low. Alternatively, in theextreme case, changes Δi affected by a single potential change ΔU can bedetermined for several current supply inputs, so that a measuringcircuit is required for each current supply input.

In accordance with a preferred embodiment of the invention, therepresentative measuring signal is determined from equations whichcontain the sum of the voltage components over the individual phasestrands prior to and after the voltage change.

In the following the invention will be explained in more detail with theaid of embodiments and the enclosed drawing which refers to theseembodiments. In the drawing:

FIG. 1 shows a schematic illustration of an electrical machine withphase strands switched in a star configuration and devices fordetermining the position of the rotor in accordance with the method ofthe invention,

FIG. 2 shows a measuring device for determining changes Δi of the timederivation i of the current I in a phase strand, and

FIG. 3 shows a schematic illustration of an electrical machine withphase strands switched in a triangular configuration, and devices fordetermining the position of the rotor in accordance with the methodaccording to the invention.

Three phase strands 1, 2, 3 of an electrical machine 4 switched in astar configuration each form an inductive resistance 5 and an ohmicresistance 6. The ends of the phase strands 1, 2, 3 facing away from thestar point 7 are connected to connections 1′, 2′, 3′ for current supplylines 1″, 2″, 3″.

A current supply device 8 connects the current supply lines 1″, 2″, 3″through switching devices 9 to 11 with the direct voltage U_(B) of abattery 12 or the voltage zero point corresponding to the pulse widthmodulation method (PWM-Method).

Measuring devices 13 to 15 in the current supply lines 1″, 2″, 3″ servefor determining changes Δi, i.e. changes of the first derivation of thecurrent I flowing through the respective current supply line over time.This refers to short changes which result from switching by theswitching devices 9 to 11.

The measurement devices 13 to 15 are in connection with an evaluatingdevice 16 which, in turn, is connected to the current supply device 8through a control line 17.

As illustrated in FIG. 2, the measuring devices 13, 14, 15 may have, forexample, a measuring transformer 18 whose secondary winding 19 deliversa measurement for i, i.e. the first derivation of the current I overtime. An amplifier 21 ensures that the inductivity of the primarywinding 20 can be small in comparison to the inductivity of the phasestrands 1, 2, 3.

With respect to the voltages U₁ applied to the individual phase strandsi (i=1, 2, 3), the following is true at any point in time:

U ₁ =U _(i ind) +L _(i) i _(i) +R _(i) I _(i)  (1),

wherein U_(i ind) denotes the voltage induced in the phase strand i,L_(i) denotes the inductivity of the phase strand i, and R_(i) denotesthe ohmic resistance thereof.

Depending on whether the battery voltage U_(B) or the voltage zero pointcontact the phase strand i, U_(i)=0, U_(i)=U_(S), U_(i)=U_(B)−U_(S), orU_(i)=U_(S)−U_(B), wherein U_(S) refers to the potential at the starpoint 7.

For example, if starting from a switching state in which all three phasestrands 1, 2, 3 are connected to the voltage zero point, the followingapplies:

0=U _(i ind) +L ₁ i ₁ +R ₁ I ₁  (2)

0=U _(2 ind) +L ₂ i ₂ +R ₂ I ₂  (3)

0=U _(3 ind) +L ₃ i ₃ +R ₃ I ₃  (4).

The ohmic resistances R_(i) can be considered as being equal and theycan be considered to be constant during motor operation of theelectrical machine. On the other hand, the inductivities L_(i) depend onthe respective influence of the exciter field and, thus, on the positionof the rotor relative to the magnetizations of the pole winding cores.Within half a magnetic period there is always an unequivocalrelationship between the inductivity of the phase strands and theposition of the rotor which can be utilized for determining theposition, as will be explained in the following.

After switching, for example, the phase strand 3 to the battery voltageU_(B) by means of the switching device 11, the following result isobtained:

U_(S) =U _(1 ind′) +L ₁ i _(1′) +R ₁ I _(1′)  (5)

U_(S) =U _(2 ind′) +L ₂ i _(2′) +R ₂ I _(2′)  (6)

U_(B) −U _(S) =U _(3 ind′) +L ₃ i _(3′) +R ₃ I _(3′)  (7).

The change of the switching state characterized by the equations (2) to(4) into the switching state according to the equations (5) to (7) takesplace so quickly that neither the voltages U_(1 ind) induced in thephase strands nor the currents I change significantly so thatU_(i ind)=U_(i ind′) and R_(i) i₁+R₁ I₁. The switch essentially only hasan effect on i_(i) ie., the first derivation of the currents over time.By subtracting (2)−(5), (3)−(6) and (4)−(7), the following is obtained.

L ₁(i ₁ −i _(1′))=L ₁ Δi ₁ =U _(S)  (8)

L ₂(i ₂ −i _(2′))=l ₂ Δi ₂ =U _(S)  (9)

L ₃(i ₃ −i _(3′))=L ₃ Δi ₃ =U _(B) −U _(S)  (10).

The values Δi ₁, Δi ₂, Δi ₃ can be determined by means of the measuringdevices 13 to 15. In the three equations (8) to (10), the inductivitiesL₁, L₂ and L₃ as well as the potential U_(S) at the star point are thenunknown.

From the three equations (8) to (10), the ratios L₁/L₂, L₁/L₃, L₂/L₃ canbe determined while eliminating U_(S) which each represent a dimensionfor the position of the rotor within half a magnetic period.

In addition, the following applies to the above described switch:

U _(B)=(l ₃+1/(1/L ₁+1/L ₂))Δi ₃  (11).

Consequently, four equations (8) to (14) are available for determiningthe unknown values L₁, L₂, L₃ and U_(S). Each of the inductivities L₁,L₂, L₃ may serve as a dimension for the position of the rotor withinhalf a magnetic period.

It is understood that for obtaining several equations from which U_(S)can be eliminated and L₁, L₂ or/and L₃ can be determined, severalsuccessive switching state changes can be considered as long as thecondition is adhered to that over the total duration of the measurementsthe position of the rotor does not perceptibly change. Under theseconditions, Δi does not have to be determined in all three phasestrands. When considering several switching state changes, themeasurement of Δi in a single phase strand may be sufficient.Accordingly, only a single measuring device is required.

Equations for determining L₁, L₂, or/and L₃ can also be set up for thesituation in which the phase strands 1, 2, 3 are switched in atriangular configuration, as illustrated in FIG. 3.

A first switching state change concerns, for example, switching of theconnection 1 a′ from the voltage zero point to battery voltage U_(B). Asecond switching state change concerns the switching of the connection 2a′ from the voltage zero point to battery voltage U_(B). This results inthe three equations

Δi ₂ L ₁ =U _(B)  (12)

Δi ₃ L ₃ =U _(B)  (13)

Δi ₃ L ₂ =U _(B)  (14),

from which the unknown values L₁, L₂, L₃ can be determined.

The measuring devices (13) to (15) in the embodiment according to FIG. 2determine, or at least one such measuring device determines, a voltagesignal S representative for the time derivation i of the current I inthe respective current supply line, wherein the voltage signal issupplied to the evaluating device 16. Control signals received throughthe control line 17 and indicated by the switching devices 9 to 11permit the determination of S prior to and after switching, and thus thedetermination of signal ΔS which is representative for Δi. Theevaluating device 16 determines from the signals ΔS, for example, withthe aid of the above mentioned equations, the signals representative forthe position of the rotor.

The above described method for determining the position could becombined with the known method which is based on the evaluation of thepotential U_(S) at the star point.

The primary winding 20 of the measuring device shown in FIG. 2 mayresult in an advantageous damping effect during the switchingprocedures.

From the determined inductivities L₁, L₂, L₃, a flow vector can bedetermined whose direction coincides with the direction of the rotorflow vector φ_(R) produced by the magnetic field of the rotor and whichis proportional to the rotor flow vector φ_(R) as long as the currentsI₁, I₂, I₃ flowing in the phase strands do not significantly influencethe total flow vector φ. The latter may be the case, especially when therotor field is weak and the air gap is large. In addition to the rotorflow vector φ_(R) which is a function of L₁, L₂, L₃, the stator flowvector φ_(S), which depends on the currents I₁, I₂, I₃, aredeterminative for the total flow vector φ:

φ(L ₁ ,L ₂ ,L ₃)=φ_(R)+φ_(S)(I ₁ ,I ₂ ,I ₃)  (15).

Consequently, in the case of known inductivities L₁, L₂, L₃ and knowncurrents I₁, I₂, I₃, the rotor flow vector φ_(R) can be computed:

φ_(R)=φ(L ₁ ,L ₂ ,L ₃)−φ_(S)(I ₁ ,I ₂ ,I ₃)  (16).

When the rotor flow vector φ_(R) is known then the rotary position ofthe rotor is also known.

The currents I₁, I₂, I₃ can be measured. However, they can also becomputed if the inductivities L₁, , L₂, L₃ are known, if the values i₁,i₂, i₃ are known (by measuring Δi₁), if the resistances R₁, R₂, R₃ areknown, and if the induced voltages U_(1 ind), U_(2 ind), U_(3 ind) areknown. They can also be computed by means of the above indicatedequations.

The voltages U_(i ind) result from the components φ_(i) of the totalflow vector φ multiplied with the determined rotary speed of the rotor.

1-12. (canceled)
 13. A method for determining a position of a rotor of an electrical machine having several phase strands, in relation to a stator, the method comprising the steps of: determining for at least one of a plurality of current supply inputs of the electrical machine, a change (Δi) of a time derivation (i) of current (I) flowing through the current supply input as a result of a change (ΔU) of a potential (U) on at least one of the current supply inputs; and, determining a measurement signal representative for the position of the rotor from several changes (Δi) obtained simultaneously or successively.
 14. The method according to claim 13, including determining as the representative measurement signal a present inductivity (L₁, L₂, L₃) of at least one of the phase strands and/or a quotient of these inductivities.
 15. The method according to claim 13, wherein the change (ΔU) of the potential (U) takes place over such a short period of time that neither voltages (U_(1 ind), U_(2 ind), U_(3 ind)) induced in the phase strands nor the currents (I₁, I₂, I₃) flowing through the current supply inputs change noticeably.
 16. The method according to claim 13, wherein the phase strands are switched in a star configuration and/or a triangular configuration.
 17. The method according to claim 13, wherein the change (ΔU) of the potential (U) takes place from a potential zero point to a direct voltage or vice versa.
 18. The method according to claim 17, wherein the direct voltage is a battery voltage U_(B).
 19. The method according to claim 13, wherein the change (ΔU) of the potential (U) takes place within a scope of a current supply of the electrical machine by pulse width modulation (PWM).
 20. The method according to claim 13, including determining the change (Δi) with a measuring transformer that delivers a voltage signal which may have to be reinforced.
 21. The method according to claim 13, including determining several changes (Δi) caused by several potential changes (ΔU) for one of the current supply inputs or by changes (Δi) caused by a single potential change (ΔU) at several current supply inputs.
 22. The method according to claim 14, including determining the representative signal from a sum of voltage components over the individual phase strands prior to and after equations containing potential changes.
 23. The method according to claim 22, including determining a present total flow vector (φ) from the present inductivities (L₁, L₂, L₃) and determining from the present currents (I₁, I₂, I₃) flowing through the current supply inputs a vector component (φ_(S)) of the total flow (φ) produced by these currents, and determining a rotor flow vector (φ_(R)) by subtracting the vector component (φ_(S)) from the total flow (φ).
 24. The method according to claim 23, wherein the present currents (I₁, I₂, I₃) are computed from the inductivities (L₁, L₂, L₃), resistances (R₁, R₂, R₃) of the phase strands, voltages (U_(1 ind), U_(2 ind), U_(3 ind)) induced in the phase strands and from measured time derivations (i₁, i₂, i₃) of the currents (I₁, I₂, I₃), and the voltages (U_(ind), U_(2 ind), U_(3 ind)) are determined from the components (φ_(i)) of the total flow vector (φ) multiplied with a measured speed of rotation of the rotor.
 25. An apparatus for determining a position of a rotor of an electrical machine having several phase strands, in relation to a stator, comprising: devices on at least one of a plurality of current supply inputs of the electrical machine, for determining a change (Δi) of a time derivation (i) caused by a change (ΔU) of a potential (U) on at least one of the current supply inputs of current (I) flowing through the respective current supply input; and an evaluating device for determining a measurement signal representative for the position of the rotor from several changes (Δi) measured simultaneously or successively. 